chapter 2: photophysical and photochemical properties

CHAPTER 2: Photophysical and Photochemical Properties Comparison of a Free Base Tetrapyridil Porphyrin

With External Ruthenium Groups.

56

which will be discussed further below. Also, the well-known singlet high energy transition,

the Soret band, was found at 413 nm. Regarding to the Q bands, the characteristic D2h

symmetry of the porphyrin in this work reflects their splitted four-banded absorption spectrum

in the Q-band region.84 All the spectral features are organized in Table 2.1.

Figure 2.2. (A) Steady-state UV-Vis absorbance spectrum of H2TPyP-Ru(terpy)PPh3 (black),

Ru(terpy)PPh3 (red) and H2TPyP (blue) in dichloromethane. The inset shows the rescaled spectra for

better visualization in the range of 350 nm to 700 nm. (B) To verify the broadening of the Soret band,

the absorbance spectra of H2TPyP-Ru(terpy)PPh3 and H2TPyP were normalized and centralized to

zero, for the sake of comparison.

Table 2.1. UV-Vis absorption spectra features of H2TPyP-Ru(terpy)PPh3, Ru(terpy)PPh3 and H2TPyP in dichloromethane.

UV-vis absorption λMAX (nm)

Ru component Porphyrin component Soret Band Q-band

Ru(terpy)PPh3 230 268 311 334 470 H2TPyP 416 510 545 587 645

H2TPyP-Ru(terpy)PPh3 230 268 311 334 470 413 510 545 587 645

In order to better understand the influence of the peripheral ruthenium groups in the

H2TPyP-Ru(terpy)PPh3 absorption spectrum, the free base tetrapyridil porphyrin, H2TPyP,

was taken for comparison, Figure 2.2A. The difference between them is remarkable in almost

all the UV and visible range. The solely H2TPyP does not show significant absorption

features at the UV region, and it also has a characteristic energy gap between the Soret band

and the Q-band, 440 – 490 nm.



57

Comparing the spectra in Figure 2A, the UV region of Ru(terpy)PPh3 has been

ascribed to intraligand π → π* transitions within both the terpyridine (terpy) and the

triphenylphosphine ligands (PPh3).81, 85-87 In the visible region, the lower energy absorption

bands were assigned to metal-to-ligand charge transfer (MLCT) transitions which are

predominantly Ru(dπ) → terpy(π*), in analogy to other reported works.88 Furthermore,

previous work have established categories to the structuration of the Q-bands based on

perturbation by external substituents.84, 89-90 Although the ruthenium moiety enhances the

overall absorbance of H2TPyP-Ru(terpy)PPh3 between 400 nm and 550 nm, by subtraction it

is possible to ensure that the intensity of the porphyrin Q-band absorption spectrum is

preserved. It’s worth noting that extinction coefficients due to the ruthenium outlying groups

were in a 4:1 proportion relative to the porphyrin ring. Consequently, the observed extra

features in the absorption spectrum of H2TPyP-Ru(terpy)PPh3, below 390 nm and in between

of 450 nm and 550 nm, were essentially attributed to the overlapping of those belonging to

the Ru(terpy)PPh3 outlying substituents. The results suggested that the external complexation

with the ruthenium groups did not significantly perturbed the porphyrin lowest singlet excited

state, S1, maintaining the phyllo type for the Q bands relative intensity.

While the Q bands did not seemed to be affected by the ruthenium groups, the Soret

band, instead, exhibited major changes that could not be explained simply by overlapping and

sum of individual contributions. It was observed significant decrease in the oscillator strength

and also the broadening of the Soret band, Figure 2.2A and B. As previously reported, the

observed broadening of the Soret band is a clear consequence of new vibrational modes being

activated after the complexation of H2TPyP with the Ru(terpy)PPh3 groups. However, up to

our knowledge, there is no established explanation to the decrease of the Soret band oscillator

strength. Similar results have also been reported.91

Such findings can indicate that the outlying ruthenium groups mainly interact with the

second excited singlet state. Probably the molecular orbitals giving rise to the Q band do not

involves the pyridine ligands while he S2 excited state may overlap orbitals of both individual

components.

PL experiments on Ru(terpy)PPh3 revealed it has two weak emission bands centered at

545 nm and 625 nm, Figure 2.3A. The substituted porphyrin exhibited very similar

photoluminescence spectral features with the unsubstituted H2TPyP,13 with peaks at 650 nm

and 711 nm. Additionally, a very weak signal was observed at wavelengths lower than 600

nm, which is unusual for most of regular porphyrins, Figure 2.3B. These extra bands were

assigned to the emission originated in the peripheral ruthenium moieties. Their very low



58

signal was due to the low concentration of the porphyrin solution used to excite directly at the

Soret band maximum absorption band, and to the significantly low photoluminescence

quantum yield of the ruthenium moiety. When overlaid and normalized to the 545 nm

wavelength, it is clearly seen that the extra photoluminescence signal was due to the

overlapping of a multiple and independent photoluminescent sites, the porphyrin ring and the

external ruthenium moieties.

Figure 2.3. Normalized steady-state photoluminescence of H2TPyP-Ru(terpy)PPh3 (black) and

Ru(terpy)PPh3 (red) in dichloromethane, excited at 470 nm. (A) normalizes the spectra at the emission

maximum peak, while (B) normalizes the spectra at 545 nm.

The hypothesis that the H2TPyP-Ru(terpy)PPh3 photoluminescence is a result of the

overlap between a stronger signal from the porphyrin ring and a second and less intense

emission originated at the Ru(terpy)PPh3 groups, could be confirmed by simply taking the

excitation spectra of the substituted porphyrin, Figure 2.4. Excitation spectra by monitoring

the porphyrin emission peak at 650 nm accurately retrieved the unsubstituted H2TPyP

porphyrin absorption spectrum. While probing the emission at 545 nm exclusively recovered

the visible range of the Ru(terpy)PPh3 absorption spectrum. This result supported the idea that

both moieties might be interacting independently upon light interaction, expressed through

their steady state photophysical properties. Although linked, both compounds did not show

intramolecular interactions such as electron transfer. Probably some minor perturbations can

be found with more specific experiments and data analysis.

Generally, heteroleptic ruthenium(II) polypiridyl complexes exhibit

photoluminescence from triplet excited states with lifetime from hundreds of nanoseconds to

tens of microseconds associated with the deactivation of thermalized tripled excited states to



59

the ground state.92-93 Time-resolved photoluminescence on Ru(terpy)PPh3 revealed that the

excited state lifetime was bi-exponential with one subnanosecond component and a second

component of 3 ns. These measured lifetimes are much faster than the expected. Most of the

heteroleptic ruthenium compounds have high quantum yield of triplet state formation and

high intersystem crossing rates.92-93 So, the nearly formed triplet excited states were rapidly

deactivated through either fast radiative or non-radiative pathways back to the ground state,

S0.

Figure 2.4. Excitation spectra of H2TPyP-Ru(terpy)PPh3 (black) probing the emission at 650 nm; and

Ru(terpy)PPh3 (red) in dichloromethane probing at 545 nm.

2.3.2 – The Absence of Intramolecular Charge Transfer Interactions.

Previous work have reported the H2TPyP excited state lifetime to be ~8 ns and well

fitted with a monoexponential decay function. This decay is associated with the depopulation

of singlet excited states back to the ground state, S0 ← S1.13 Here, when complexed with the

Ru(terpy)PPh3 groups, the H2TPyP-Ru(terpy)PPh3 porphyrin reveals now a fast,

subnanosecond, characteristic lifetime in additional to the well known unsubstituted free base

porphyrin lifetime, Figure 2.5. The performed time-resolved photoluminescence experiment

revealed that the H2TPyP-Ru(terpy)PPh3 excited state is deactivated predominantly (70 %) by

a fast subnanosecond process, τ1 = 0.4 ns, while the regular H2TPyP excited state lifetime of 8

ns contributes to 30 % of its depopulation. The origin of such a faster component might be

related to the enrichment of non-radiative pathways back to the ground state resulted from

extra vibrational modes. Such possibility is corroborated by the fact that the insertion of the



60

outlying groups implied in an enlargement of the Soret-bandwidth, which can be explained as

being due to the creation of new vibrational modes, favoring the dissipation of energy in

nonradiative ways.

Additionally, the calculated photoluminescence quantum yield of 0.45% is 3.8 times

smaller than that reported for the unsubstituted H2TPyP (1.7%).13 This latter observation

shows that the external ruthenium groups worked as quenchers to the porphyrin excited state,

where the quenching mechanism might be strongly coupled with the induced new vibrational

modes. The time-resolved parameters and photoluminescence quantum yields are shown in

Table 2.2.

Figure 2.5. Time-resolved photoluminescence of H2TPyP-Ru(terpy)PPh3 (open circles) and

Ru(terpy)PPh3 (solid circles) with excitation wavelength at 460 nm, with their respective multi-

exponential fitting.

Table 2.2. Multi-exponential parameters an emission quantum yield of H2TPyP-Ru(terpy)PPh3 and Ru(terpy)PPh3 in dichloromethane.

Compound τ1 (ns) τ2 (ns) ϕ (%)

H2TPyP-Ru(terpy)PPh3 0.4 (0.67) 7.9 (0.33) 0.45

Ru(terpy)PPh3 0.64 (0.61) 3.0 (0.39) -----

The values in parentheses denote the associated weight of each lifetime.

Nanosecond transient absorption experiments for a great variety of porphyrins have

mainly revealed the triplet state dynamics.13-15, 71, 75 The transient absorption spectrum of a

free base tetrapyridyl porphyrin shows a characteristic bleach of the ground state between 400

nm and 430 nm, and a growth from 430 nm to 500 nm. In transient absorption, a growth

signal represents an increase in absorption, and generally for porphyrins, this is assigned to

the triplet excited state absorption, T1 → Tn, Figure 2.6. The results showed clear similarity



61

between the transient absorption spectral shape of H2TPyP and H2TPyP-Ru(terpy)PPh3

spectra. Although, in a closer analysis, some differences could be pointed out: 1) For

H2TPyP-Ru(terpy)PPh3 the ground state bleach of the Soret band was broader and 2) its

relative intensity (with the growth peak) was smaller, 3.5, against 5.7 for H2TPyP; 3) the

kinetic traces for all probed wavelengths were much longer lived for H2TPyP-Ru(terpy)PPh3

than it was for the unsubstituted porphyrin. While the first two observations were connected

with spectral shape of the stead state UV-vis absorption spectra, the later told us that the

ruthenium groups changed the kinetics of the porphyrin decay pathways in both the singlet

excited state previously discussed, and triplet excited states.

Figure 2.6. (A) Transient absorption spectra obtained over a 30 ns – 50 µs time window after pulsed

532 nm excitation of H2TPyP-Ru(terpy)PPh3 in dichloromethane. (B) Single wavelength transient

absorption kinetic data of H2TPyP-Ru(terpy)PPh3 in dichloromethane, observed at 413 nm. Overlaid

in red is the fit to the monoexponential function with τ = 9 µs. (C) and (D) represent the transient

absorption spectra and the single wavelength transient absorption kinetics of the unsubstituted H2TPyP

porphyrin in dichloromethane. Overlaid in red is the monoexponential fit given a decay lifetime of 0.6

µs.



62

The measured transient triplet excited state lifetime were about 15-fold longer for

H2TPyP-Ru(terpy)PPh3 (9 µs) than it is observed of H2TPyP (0.6 µs). Previous work13 have

reported that the peripheral complexation of H2TPyP with ruthenium groups is deleterious to

the triplet state population, S1 → T1, evidenced by an increase in the intersystem crossing

time, τISC. The later behavior is contrary to that observed when a metal replaces the hydrogen

atoms to stabilize the porphyrin ring. Also, in the current work, an expressive increase in the

triplet state lifetime, τT, is observed when a ruthenium triphenylphosphine is linked to the

pyridines of the H2TPyP. Specific experiments to abstract τISC were not performed in this

work, but the trend in the increase of τT after the ruthenium complexation was observed. Also,

the fact that 70% of the S1 excited state deactivates via a subnanosecond process, is an

indirect probe to reinforce that the triplet excited state was substantially quenched by faster

deactivations pathways from the first singlet excited state. It wasn’t observed transient

absorption signal for Ru(terpy)PPh3 under same experimental conditions and on the

nanosecond time scale.

Finaly, cyclic voltammetry was performed on the two compounds presented in this

work with applied reserve (positive) bias, Figure 2.7. The Ru(terpy)PPh3 presented formal

reduction potential, E0, of 1064 mV vs NHE, where the observed waves were consistent with

the formation of RuIII upon oxidation of RuII. The free base tetrapyridyl porphyrin containing

the peripheral ruthenium groups showed very similar result as the Ru(terpy)PPh3. The

reduction potential was measured to be 1132 mV vs NHE and again was assigned as the

equilibrium potential (E1/2) where the concentration of RuIII and RuII were equal, rather than

the oxidation of the porphyrin ring. The comparison of the two cyclic voltammetry revealed

that the porphyrin ring weakly affects the overall redox properties of the ruthenium groups.

The more positive Ru(III/II) reduction potentials of the H2TPyP-Ru(terpy)PPh3 molecule,

when compared with the single Ru(terpy)PPh3 complex, suggested that the electron-

withdrawing characteristic of the pyridines stabilize the dπ levels of the Ru2+, lowering down

its orbital energy. It is known that the energy necessary to remove one electron from the

molecule ground state configuration (i. e. to oxidize it) is directly associated with to HOMO

energy.

Oxidation of the porphryin ring was not observed in cyclic voltammetry, even at

further positive potentials. Performing DFT calculations to obtain the porphyrin HOMO

energy, which could simulate the ring first oxidation potential, approximately fulfilled this

limitation. The calculated value was at about -6 eV, relative to the vacuum energy.



63

Figure 2.7. Cyclic voltammetry of H2TPyP-Ru(terpy)PPh3 (black) and Ru(terpy)PPh3 (red) in Ar-

purged 0.1 M TBA(PF6) dichloromethane electrolyte solution.

The reduction potential diagram, Figure 2.8, is an important and complementary tool

when studying electron transfer processes. Associated with transient absorption experiments,

the diagram can predict the existence or absence of electron transfer. The following diagram

was constructed using cyclic voltammograms of H2TPyP-Ru(terpy)PPh3 in 0.1M TBA(PF6)

electrolyte solution in dichloromethane, in which the formal reduction potentials of oxidation

and reduction of the ruthenium moiety were obtained. With regards to the porphyrin ring, its

reduction potential as an electron acceptor was abstracted from cyclic voltammetry while the

ring oxidation was estimated from the DFT calculated HOMO energy of a free base

tetrapyridyl porphyrin.

Figure 2.8. Reduction potential diagram for electron transfer processes of H2TPyP- Ru(terpy)PPh3.

The black lines are potentials associated with the porphyrin ring while the red lines are the potentials

for the Ru(terpy)PPh3 moiety, in dichloromethane.



64

Transient absorption kinetics of H2TPyP-Ru(terpy)PPh3 showed mono-exponential

behavior and the spectra were fully normalized, results that converge to the single transient

process of triplet excited state formation. The spectral shape observed in Figure 2.6 have

similar characteristics reported in many other works.15, 94-95 In the reduction potential energy

diagram, Figure 2.8, electron transfer from the porphyrin ring to the ruthenium complex is an

uphill reaction and thus not favorable. Electron transfer from Ru(terpy)PPh3 to the porphyrin

ring has very small driving force, which, from Marcus theory for electron transfer processes,

would be associated to small rate constants, or longer life-times.96 Excited state deactivation

of Ru(terpy)PPh3 , Table 2.2, might be fast enough to efficiently kill electron transfer

processes.

2.4. CONCLUSIONS

In this work, was performed steady state and time-resolved spectroscopy, and

electrochemical experiments to compare the photophysical properties of the free base

tetrapyridil porphyrin (H2TPyP) when ruthenium groups are externally linked to it. Stead state

spectroscopy revealed that absorption and emission spectra were independent to their specific

origins, the H2TPyP porphyrin moiety and the Ru(terpy)PPh3 complex. However, when time-

resolved experiments were employed, significant changes were observed in the H2TPyP-

Ru(terpy)PPh3 porphyrin photophysical properties. It was seen that the complexation creates

new vibrational modes between the first singlet excited state and the ground state that can be

easily accessed through a fast deactivation process. The later suggests that triplet excited

states were strongly quenched by the presence of outlying ruthenium groups. Also, as seen in

previous work13, the triplet excited state lifetime, τT, suffered an increasement when compared

with the unsubstituted H2TPyP. Electrochemical experiments provided that the ruthenium(II)

(dπ) orbital is stabilized when linked to the pyridines of the H2TPyP, evidenced by a 68 mV

vs NHE shift to less positive potentials of the Ru(III/II) reduction potential. Finally, although

the H2TPyP-Ru(terpy)PPh3 compound could have strong light harvesting, the absence of

intramolecular charge transfer between the two groups is not favorable to sensitization in

DSSCs. The results found in this work revealed good insights about appropriate molecular



65

engineer linking porphyrins and ruthenium compounds. The presence of metal ions as central

substituent in the porphyrin ring could modify the porphyrin energy levels and excited state

dynamics so that the porphyrin ring can communicate more efficiently with the outlying

ruthenium groups. If succeeded, such modified molecular structures could be a good

candidate for better light harvesting and photoinjection in DSSCs.

66

PART II: Sensitization of Mesoporous TiO2 Thin Films With a Free

Base Tetrapyridyl Porphyrin With Peripheral Ruthenium

Groups.

INTRODUCTION – PART II

67

INTRODUCTION

The Dye Sensitized Solar Cell

Since the prediction that fossil fuels would be completely dried up in a near future, an

emerging need for new sources of energy started to motivate researches to pursue potential

alternatives to fulfill the world energy needs. The necessity to develop economically viable

renewable energy sources is fundamental to achieve sustainability in a globally society. The

global energy consumption in the year 2000 was 13 TW and estimatives predicts this value to

be 28 TW in 2050 for the global energy demand.97-98 The solar energy is definitely one of the

largest potentially promising source to satisfy the future global need for renewable energy

sources, since 1.7 × 105 TW of solar energy strikes the Earth’s surface per year.99 If 0.13% of

the Earth were covered with solar cells with an efficiency of 10%, the generated energy would

satisfy our present needs.100

At the end of last century, the possibility to develop photoelectrochemical cell devices

for solar electricity production based on molecules as light harvest agent seemed unlikely to

happen. This concept started to change when O’Regan an Grätzel proposed, in 1991, the

clever idea of utilizing mesoporous thin films of nanocrystalline TiO2 sensitized with dye

molecules in photoelectrochemical cells, Figure II-1.101 The idea was the starting point to

encourage researches on this ascending new research field of the dye sensitized solar cells

(DSSCs). The prospect of low-cost investments and fabrication are some of its key features.

Today, with the development and intense research on the DSSC, there are records on devices

with 13% efficiency, with high stability.1 The increasing efforts to optimize DSSC devices

establish a major challenge to the conventional solid-state photovoltaics technologies. While

solid-state photovoltaics demands special laboratories to produce highly pure semiconductor

materials, avoiding defects and interfaces, the DSSC, in contrast, is based on interface

interactions (oxide/dye/electrolyte) and its preparation could be achieved in simple laboratory

environment.


68

Figure II-1. Atomic Force Microscopy (AFM) of a mesoporous thin film of anatase TiO2

nanoparticles. Mesoporous thin films are about 10 µm thick and each TiO2 nanoparticle has

approximately 20 nm diameter.43

The typical basic processes involved in a dye sensitized solar cell (DSSC) showing the

principle of how the device operates are schematically depicted in Figure II-2, and can be

found in many reviews in the area.44, 102 At the heart of the device are the sintered TiO2

nanoparticles (10 – 30 nm) interconnected in a mesoporous oxide thin layer (~10 µm and 50%

porosity) to establish electronic conduction. The mesosporous thin layer is then deposited on a

transparent substrate, which is commonly used the glass coated with fluorine-doped tin oxide

(FTO). Anchored to the nanocrystalline film surface are the charge-transfer dye molecules

working as sensitizers for light harvesting, see Figure II-3. Upon illumination, such sensitizers

undergo to photogenerated excited states in a few femtoseconds resulting in ultrafast electron

injection into TiO2, on the subpicosencond time scale. After injection, the dye is left in its

oxidized state due the loss of one electron. The sensitizer is regenerated to its ground state by

electron transfer from the electrolyte, usually an organic solvent (commonly acetonitrile)

containing the iodide/triiodide redox system. The regeneration step works on the microsecond

domain, which effectively annihilates the oxidized dye molecules S+ and intercepts the

recapture (recombination) of the conduction band electrons with S+, which happens in the

millisecond time range. After I- regenerates the oxidized dyes, the left I• radicals quickly

reacts with others I- ions to form the I3-. The formed triiodide ions diffuse through the

electrolyte to the counter electrode, which is coated with a thin layer of platinum catalyst.

Finally the regenerative cycle is completed when I3- is reduced back to I- by electron transfer

from the cathode.


69

Besides the desired electrons transfer process described above, there are undesired

losses of energy through side reactions. Some of them are the direct relaxation of the dye

excited state to its ground state defined by the excited state life time. Another major loss of

energy in DSSC is the back reaction of the injected electrons in the TiO2 with either oxidized

dyes or acceptors redox mediator in the electrolyte. Figure II-2, depicts the general reaction

processes in dye sensitized solar cells.

Figure II-2. Scheme of the possible reactions of charge transfer in a DSSC.

Energetic Features of the DSSCs

Figure II-3. Overview of processes and typical time constants under working conditions in a

ruthenium-based dye solar cell with iodide/triiodide electrolyte. Recombination processes are

indicated by red arrows.


70

Besides the attention to the limits in terms of kinetics, the energetics of DSSC is of

fundamental importance to the function of the device, Figure II-3. This energetics comprises

the positions of the energy levels at the oxide/dye/electrolyte interface, such as the energy

levels for semiconductors, redox couples, and dye reduction potentials adsorbed to surfaces.

Before getting into details for the understanding of Figure II-3, it is necessary to make

clear some important aspects of the concepts in energetics of the DSSCs. The diagram is a

clear example of the interdisciplinarity created between physics and chemistry. While solid-

sate physics normally uses the energy scale relative to the vacuum energy level, the

electrochemistry has the potential scale with the standard hydrogen electrode (SCE) or the

normal hydrogen electrode (NHE) as reference to the energy scale. For semiconductors, the

electrochemical potential of electrons is commonly associated as the Fermi level, EF, and

systems in an electrolyte solution, it is normally referred to in potential as the redox potential,

Eredox. The later basically represent the equilibrium potential where the concentrations of the

redox states are equal. An estimated connection between the potentials of the NHE versus an

electron in vacuum is given in Equation II-1.103

𝐸! 𝑒𝑉 = −4.5 𝑒𝑉 − 𝑒𝐸!"#$%[𝑉] (II-1)

where e is the elementary charge. Most charge transfers in DSSC are limited to one electron

reactions, which makes the conversion from Volts (V) to electronVolts (eV) straightforward.

For semiconductor, the equilibrium Fermi level is very sensitive to the environment

and the determination of its absolute and relative energies in vacuum must be carefully

analyzed in terms of their relevance to DSSC. Many electrochemical, photochemical, and

spectroscopic studies have reported that mesoporous, nanocrystalline TiO2 thin films possess

several acceptor states tailing below the TiO2 conduction band, Ecb, rather than an abrupt

onset from an ideal Ecb.43, 104 While there is emergent the belief that a well defined Ecb is not

relevant to excited state injection it was found that the density of states (DOS) distribution at

room temperature is of fundamental relevance to the functioning DSSC. At room temperature,

this distribution is thought to have an exponential dependence on the applied voltage as

determined from electrochemical105-107 and spectroelectrochemical procedure108. The tail

governed by an exponential dependence with the increase in energy arises from what is

thought to be imperfections in the TiO2 particle, where trapped states are energetically

favorable to accept electrons.43, 105, 109-110 The equilibrium Fermi level described by Boltzmann


71

statistics can be rearranged to isolate the cumulative number density of conduction band

electrons at a specific energy, E.

𝑛! 𝐸 = 𝑁! exp !!!!!∙!!!

(II-2)

where Nc is the effective density of conduction band states, Ec is the energy at the conduction

band edge, kBT is the thermal energy and 𝑎 is the non-ideality factor. The non-ideality factor

represents the deviation from the expected 59 mV increase in energy per decade of TiO2

electron concentration. Additionally, the non-exponential kinetics for excited-state electron injection can be

rationalized supported by the exponential DOS at the TiO2 surface.111-114 And by assuming

said distribution is composed of bulk, intra-bandgap states, diffusion of TiO2 electrons, and

dispersive recombination kinetics, the general kinetics for excite-state electron injection can

be modeled satisfactorily by employing the continuous-time random walk model,112, 115-119

which simulates the multiple-trapping detrapping events for charge recombination. The model

is based on principles of dispersive transport where the waiting times for electrons motion are

directly related to the energetics distribution of trap states.117

Besides the favorable kinetics for each charge transfer reaction, the energetics of the

system is of fundamental importance. Dye molecules in its ground state have such positive

potentials for oxidation that it would be impossible to initiate an electron transfer to the

acceptor states of the TiO2. However, when a sensitizer is photoexcited, it’s more easily

reduced or oxidized, because of the excitation free energy ∆GES stored in thexi state of the

molecule. Electron injection will only occurs if the dye reduction potential at the excited state

is above the distribution of TiO2 acceptor states. Similarly, redox mediators, such as the I-/I3-

couple can only regenerate the oxidized dyes it can access the reduction potential of the

sensitizer ground state.

The open circuit voltage, Voc, represents the maximum energy that can be extracted

from the system. It is the maximum Gibbs free energy that can be harnessed during steady

state irradiance. The magnitude of Voc is measured to be the difference between the quasi

Fermi-level of the semiconductor and the redox couple, Figure II-3. Although the addition of

cations, such as Li+, in the electrolyte solutions aim to lower in energy the TiO2 density of

acceptor states to increase electron injection after photoexcitation, it is unfavorable to the Voc

once it would directly lower the energy of the quasi-Fermi level. The DSSC system


72

optimization in order to get higher Voc values is a research field of major interest trying to

find better redox mediator with more positive potentials and the molecular engineering trying

to find dyes increasingly suitable for charge injection without the need of cations additions.

Photoinduced Electron Injection

DSSC have a unique feature when compared with other solar cell technologies, its

charge separation state spatially separates the light absorber from the charge carrier

transporter (for both electrons and holes), in other words, the two processes are independent

and happen at different materials.

As previous discussed, the non-exponential kinetic for electron injection into TiO2 is

generally found to result from the heterogeneity of TiO2, and its many consequences such as

intra band-gap, trap states, electron diffusion into TiO2, combined with the sensitizer binding

modes and strengths, interactions between sensitizers and possible aggregates, and multiple

ultrafast injection processes occurring from various states in the thermal relaxation pathway.

The charge separation mechanism have been extensively studied in the past years, and it’s

believed injection can take place from multiple excited states in the thermal relaxation

pathways with distinct rates.38, 43, 120-121 More often, the injection pathways discussed are from

the initially formed Franck-Condon singlet excited state (~ fs),121 the thermally relaxed singlet

excited state (~100 fs) and finally from the thermally relaxed triplet excited state after

intersystem crossing from singlet excited state (~ ps).38 The first and faster pathway is often

called “hot injection” and is highly dependent on the excitation wavelength, TiO2 Fermi level,

and the distance between sensitizer and the semiconductor’s surface. The slower component

from triplet excited states in the picosecond regime results in biphasic injection kinetics.122

Extensively used in DSSCs, ruthenium based sensitizers like N3, N719 and black dye,

have strong light absorption from and metal-to-ligand charge transfer (MLCT) state. The

MLCT character promotes electrons, after excitation, from the metal center to the

carboxylated bipyridyl ligand that is directly anchored to the semiconductor surface. In this

fashion, excited-state charge separation occurs from the π* orbitals of the organic ligand of

the ruthenium complex to the acceptor states in TiO2. The injection process for such

ruthenium base dyes has been extensively debated. Ultrafast injection with time constants


73

>100 fs have been measured which would imply that injection is occurring before thermal

relaxation of the molecular excited state.43, 123 The slower component, in the picosecond

range, is discussed as an injection mechanism proceeding via the lower energy triplet excited

state. The spin-orbit coupling from the ruthenium heavy atom center enhances the probability

for spin state changing and it gives the time for intersystem crossing to occur at hundreds of

femtoseconds.38

Figure II-4. Lennard-Jones potential energy diagram illustrating the relative electronic and vibrational

energy and lifetime for a polypyridyl ruthenium(II) complex. Internal-conversion (2) and intersystem

crossing (3) happens in the sub-picosecond time scale. The lifetime of the thermalized excited-state

ranged on the nanosecond to microsecond time scale.

The theory for excited state electron injection into wide-bandgap semiconductor was

first introduced by Gerischer.36, 124-125 According to this theory, the rate of interfacial electron

transfer at an electrode surface is proportional to the overlap of occupied electron donor with

unoccupied acceptor states. Thus, in DSSCs, the rate constant and efficiency for electron

injection critically depends on the overlap between the sensitizer excited-state distribution

function and the semiconductor density of states (DOS).

𝑘!"# ~ 𝜅 𝐸 𝐷 𝐸 𝑊!"# 𝐸 𝑑𝐸 (II-3)


74

where E is the energy, 𝜅 𝐸 is the transfer frequency factor, 𝐷 𝐸 is the density of

unoccupied acceptor states (DOS) in the semiconductor (electron acceptor), and 𝑊!"# 𝐸 is

the sensitizer donor distribution function (electron donor).

Regeneration of the Oxidized Dyes.

After photoinduced electron injection form the dye into the TiO2 DOS, the dye is left

in its oxidized state and an electron donor in the electrolyte for regeneration must reduce its

ground state. In DSSCs, redox mediators are added to the external electrolyte. The reduced

form of the mediator must regenerate the oxidized sensitizer by electron transfer prior to

recombination with the injected electron. Then, the oxidized form of the redox mediator is

reduced at the platinum counter electrode. Ideally, all redox states of the redox mediator

would not competitively absorb light. By far the most effective donor in DSSCs is iodide.126

In summary, the iodide/triiodide couple has good solubility, does not absorb too much light,

has a suitable redox potential, and provides rapid dye regeneration. But, its major advantage is

the very slow recombination kinetics between electrons in TiO2 and the oxidized form of the

redox mediator, the anionic triiodide (I3-), which could be associated with the electrostatic

repulsion among the species.127

The regeneration of oxidized dyes with iodide leads to the formation of the diiodide

radical (I2-•),128-131 observed for many groups using nanosecond transient absorption and

pseudo-steady-state photoinduced absorption spectroscopy.132 The pathway for dye

regeneration by iodide is given by the reaction in Figure II-5.43, 130, 133

Figure II-5. Scheme for the overall mechanism for sensitizer regeneration, where the I-/I3

- work as the

redox mediator.


75

After electron injection from the excited dye S*, the oxidized dye S+ is reduced by

iodide, under the formation of the complex (S⋅⋅⋅I). The oxidation of iodide to free iodine

radical (I•) is unlikely from energetic reasons: E0(I•/I-) is +1.33 V vs NHE in aqueous

solution,134 and has been reported to +1.33 V vs NHE in acetonitrile,135 which is more

positive than the reduction potential, E0(S+/S), for most of the dyes used as sensitizers in

DSSCs. Unlikely, the redox potential of the iodine radical bound to the dye (S⋅⋅⋅I) is less

positive in potential, so that its formation is expected to be the first step in the dye

regeneration. The presence of a second iodide leads to the formation of (S⋅⋅⋅I2-•), which can

dissociate into the regenerated dye at the ground state, S, and I2-•. Finally, I2

-• can

disproportionate to produce triiodide and iodide. Nanosecond transient experiments have

reported values for complete regeneration of the oxidized sensitizers on the microsecond

scale, ~1 µs.34, 37, 133, 136 The overall mechanism for the sensitizer regeneration is depicted in

Figure II-5.

Charge Recombination: Energy Loss Mechanisms in the DSSC

A series of recombination reactions may harmfully compete with the desired reactions

in the DSSC. Radiative and nonradiative decay from the thermalize photoexcited dye is a

drain side reaction for electron injection. After photoinduced injection, electrons can undergo

back reactions, recombination, with the oxidized sensitizers on the semiconductor surface.

Moreover, injected electrons diffuse through the mesoporous TiO2 nanocrystalline network

before reaching and get collected by the conducting substract. Meanwhile, electrons may

always be near the interface between TiO2 surface and electrolyte becoming suitable to

recombine with electron acceptors in the electrolyte. Because the Voc is directly related with

the electron concentration in the TiO2 DOS undesired charge recombination processes the

maximum Gibbs free energy that can be harnessed from the system can be compromised.

Quantitatively speaking, charge recombination with oxidized dyes have been widely

studied over the last two decades, relative to the DSSC field.34, 43, 116 Two models proposed by

Nelson and Tachiya have been noteworthy for the observed charge recombination kinetics.

The first one is established on the continuous-time random walk model (CTRW), where the

transport of electrons is limited by a power law distribution of waiting time for electron


76

hoping between localized trapped states in the TiO2 in a neighboring or nearby localized

state.34, 117 In this model, it was showed that recombination is limited by multiple trapping

states of the TiO2(e-) gives rise to stretched exponential decay, when trap states are

exponentially distributed in energy.116 Later, Tachyia et al. expanded the CTRW multiple-

trapping model, proposed by Nelson et al., to account for the possibility of many neighbor

interactions, where electron can undergo multi-trapping and detrapping events through any

available trap state in the nanoparticle as opposed to only nearest neighbors.116-118

Despite the difference in theory, both models reduce their decay kinetics to the

stretched exponential Kohlraush-William-Watts (KWW) function. This function satisfactorily

model the distribution of rate constants that were originated on the waiting time distribution

of each trap-detrap event, predicted on the kinetics of the CTRW model.

𝐴 𝑡 = 𝐴! 𝑒𝑥𝑝 − !!!

! (II-4)

where A0 is the initial absorption change, 𝜏! is the characteristic lifetime,137 𝛽 is inversely

related to the width of the underlying levy distribution of rate constants, 0 < 𝛽 < 1. The

stretched exponential dispersion parameter 𝛽 is related to the exponential distribution of trap

states in the TiO2, and also depends on the electrolyte composition116 and the nature of the

dye.138 A mean lifetime for the kinetic process can be calculated from the first moment of the

KWW function,

𝜏 = !!!Γ !

! (II-5)

where Γ(x) is the Gamma function.35, 82, 139

It is noteworthy to say that not only electron recombination kinetics to oxidized dyes

are well modeled to the KWW function, but likewise charge transfer recombination to the

oxidized form of the redox mediator can be satisfactorily rationalized to the KWW stretched

exponential function.35, 43


77

Ruthenium Polipyridyl Complexes in DSSC

Ruthenium(II) polypyridyl coordination compounds, among the metal complexes,

have proven to show as most efficient sensitizers for solar harvesting and sensitization of

wide-bangap TiO2 semiconductor material,43-44, 82, 111, 140-142 a consequence of their promising

photovoltaic properties: a broad absorption spectrum, suitable excited and ground state energy

levels, relatively long excited-state lifetime, and good electrochemical stability. The

sensitization of the TiO2 nanoparticles requires the Ru complexes to have functional groups,

for anchoring activity, such as the carboxylic acid.111, 143

Figure II-6. Molecular structure of the ruthenium(II) based complexes used as sensitizers in DSSCs.

Picture extracted from reference [44].44

In 1993, Grätzel and co-workers first introduced the cis-(SCN)2bis(2,2ʹ′-bipyridyl-4,4ʹ′-

dicarboxylate)ruthenium(II), better known as the N3 dye.44, 144 It was revealed to have

outstanding properties, such as broad absorption band in the UV-vis spectrum and photon-to-

current conversion efficiency up to 800 nm, sufficiently long lived excited state (~20 ns), and

strong adsorption on the TiO2 surface. The N3 overall favorable properties granted it a solar-

to-electric energy conversion efficiency of 10%, being the first time such efficiency is

achieved for ruthenium(II) polypyridyl compounds.

Later, Nazeeruddin et al.145 investigated the effect of protonation of the N3 dye

anchoring groups on the performance of DSCs. The doubly protonated form,

(Bu4N)2[Ru(dcbpyH)2(NCS)2], coded N719, exhibited an improved power conversion


78

efficiency. The N3 and N719 dyes are considered as reference dyes for DSC and are used as a

base for designing other Ru photosensitizers by changing ancillary ligands.

Seeking improvement in the efficiency of DSSCs, researches tried to extend the

spectral light absorption of the sensitizer to the near-IR region, and many efforts have been

made to change the ligands of the Ru complexes in order to accomplish such task. Therefore,

Grätzel and co-workers designed the N749 dye, called the “black dye”.146 The study showed

that the proposed molecular structure MLCT band red shifted due to the decrease in the π*

level of the terpyridine ligand and an increase in the energy of the metal orbital. The tuning to

extend the light harvesting of the sensitizer led the achievement of photo-to-current

conversion efficiency over the whole visible range covering into the near-IR region up to 920

nm and remarkable efficiency of 10.4%.146

Although ruthenium-based dyes are the most common sensitizers implemented in

DSSCs, they have critical drawbacks. Ruthenium is not abundantly found in Earth, and it is

unlikely the current supply will support the world energy needs if such ruthenium dyes

DSSCs can be widely adopted for the realities of a solar-based economy. To overcome this

challenge, extensive research has been done to substitute ruthenium sensitizers for

inexpensive dye and easy to synthesize, while showing high efficiency.

Porphyrins as Sensitizers in DSSC

Porphyrins and phthalocyanines are widely known due to their primary role in

photosysthesis and the ease with which their photophysical and photochemical properties can

be tuned via molecular engineering. The use of porphyrins in DSSC was first introduced by

Kay and Grätrzel in 1993,10-11 and since that time the search for new efficient pophyrin dyes

for DSSC have been strongly explored through molecular engineering. One of the

disadvantages of ruthenium complexes is the limited absorption in the near-infrared region of

the solar spectrum. Porphyrins and phthalocyanines exhibit intense spectral absorption bands

in the near-IR region and possess good chemical, photo, and thermal stability, becoming

potential candidates for sensitizers in DSSCs.

These porphyrins exhibit long-lived (>1 ns) π∗ singlet excited states and depending on

the central substituent, they show weak singlet/triplet mixing. They have an appropriate


79

LUMO level that resides above the conduction band of the TiO2 and a HOMO level that lies

below the redox couple in the electrolyte solution and very strong absorption of the Soret

band in the 400-450 nm region, as well as the Q-band in the 500-700 nm region.13, 15-16, 147-149

Several studies have demonstrated that porphyrin dyes can show efficient photoinduced

electron injection into the conduction band of TiO2150-152 and that the oxidized state of the

porphyrin dyes are mainly delocalized over the conjugated macrocycle ring.151

Based on the porphyrin structure there are two main points of attachment for TiO2-

binding groups, the meso and β positions, Figure II-7. DSSCs using porphyrins with meso-

substituted benzoic acid binding groups (phenyl groups with carboxylic acid radical) are some

of the earliest and still probably the most studied of the porphyrinoid-based sensitizers.1, 149,

153

Figure II-7. On the left, it is shown the general structure for the free base porphryin ring with the two

main points of attachment for TiO2 binding groups, the meso positions (Lmeso) and the β positions (Lb).

On the right is the molecular structure the well known meso-substituted free base porphyrin with

benzoic acid binding groups.

Although porphyrins have strong absorption in the UV and visible range of the

electromagnetic spectrum, the absorptivity of this class of molecules as sensitizers in DSSCs

is limited in the red an near infrared regions of the solar spectrum. Generally, meso-

substituted porphryins have their outlying groups considered perpendicular to the plane of the

macrocyclic ring and therefore are not in conjugation with the porphyrin π-conjugated system.

To overcome such limitation, attempts to modify the meso-substituents by substitution at the

macrocycle β-position with function groups have been widely employed to extend the π

system to enhance the red-absorbing Q-bands.154 The use of conjugated β-substituted

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Copyright © 2010 World Scientific Publishing Company J. Porphyrins Phthalocyanines 2010; 14: 776–792

776 M.G. WALTER ET AL.

[102, 103], and porphyrins by themselves [104]. Based on the structural diversity of porphyrinoid sensitizers a convenient way of organizing different dyes within a type is to distinguish between the placement and structure of binding groups used to adsorb the dye to the TiO2 sub-strate. The different positions of some typical dye-linking groups within each type can be seen in Fig. 17, where L represents the linker. This approach will be used in this discussion to organize the different groups and is espe-cially relevant for the porphyrin-based DSSCs, where the location and role of the linker group has been shown to be particularly important.

As can be seen from the complexity of DSSC con-struction there can be many ways to achieve optimization and improved efficiencies, including different selections and processing of the semiconductor, electrolyte, surface treatments, solvents, and miscellaneous additives. In each case much research has been done to manipulate each variable and in some cases an entire review paper could be written just on one aspect of cell construction. Our focus will be on the specific dyes – their structures and their performance in standard DSSCs (TiO2 nano-particles on FTO glass, platinum counter electrode, and iodide/triiodide electrolyte). Unless it appears unusual or relevant, little attention will be paid to differences in cell construction techniques and additives as these will vary greatly between research groups.

Porphyrins in DSSCs

The use of porphyrins in DSSCs was first demonstrated by Kay and Grätzel in 1993 [99, 100], and since that time the search for efficient porphyrin dyes for DSSCs have been a major focus of research. Since 2000, at least 378 publications have focused on the use of porphyrins in solar cells and the trend has been consistently growing (Fig. 18).

Based on the porphyrin structure there are two main points of attachment for TiO2-linking groups, the meso and positions (Fig. 17). Within the meso-position two main classes of linking moieties have been employed most often, the classic meso-substituted benzoic acid linking and the direct attachment of a meso-alkynylben-zoic acid moiety, a much newer focal point for porphy-rin DSSC research. For DSSCs employing a -position

linking group only one clear distinction between dye attachment types can be made, the difference between conjugated and non-conjugated linkers. Conjugated link-ers have received the most attention by far and as such the attention in this discussion will be focused on con-jugated attachment methods. Since the majority of work on porphyrin DSSCs is focused on these three areas, they became natural delineators for the following compila-tion. Additional miscellaneous design strategies include other meso-based attachments, metal-ligand binding, porphyrin:C60 aggregates that bind to TiO2, and design strategies that lead to solid-state DSSCs (DSSCs that lack the classic liquid electrolyte). Throughout, we use the same abbreviations for structures as appeared in the original literature to facilitate locating the compounds in the original literature.

Porphyrins anchored through meso-phenylcarboxy groups. DSSCs using porphyrins with meso-substituted benzoic acid linking groups are some of the earliest and still probably the most studied of the porphyrinoid-based sensitizers. A general structure for a meso tetrasubsti-tuted porphyrin is given in Scheme 1. If R1-R4 are para benzoic acid moieties and M = 2H, we have 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP), which has been studied as a model system in solar cells and as a

N

NH N

HN N

NH N

HN N

HNN

NHN

NH

N

N

N

HN

N

N

Lb

Lmeso Lmeso

Lb

Porphyrin Chlorin Phthalocyanine Corrole

L

L

Fig. 17. General structures for the four main porphyrinoid light-harvesting dyes. A fifth dye type, the bacteriochlorin, is a chlorin in which two pyrrole rings opposite one another are reduced. L represents possible linker sites for attachment to the DSSC

Fig. 18. Number of porphyrin solar cell publications per year found via the search term “porphyrin solar cells” in SciFinder (June 19, 2010); note that not all reports are necessarily of porphyrins in DSSCs

00268.indd 776 12/2/2010 2:12:41 PM

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Copyright © 2010 World Scientific Publishing Company J. Porphyrins Phthalocyanines 2010; 14: 778–792

778 M.G. WALTER ET AL.

DSSC. As a part of their study, Imahori et al. optimized the adsorption time and adsorption solvent for dye loading on TiO2 films and determined that lower adsorption times and protic solvents (methanol) led to the best efficiencies. In more recent work, integration of a push-pull structure with a trans diphenylamino substituent gave a very high efficiency of 6.5% (Fig. 19, mono-ZnP) [121].

One issue with porphyrins as sensitizers in DSSCs is their limited absorptivity in the red and near infrared regions of the solar spectrum. With meso-tetraphenylpor-phyrins the phenyl moieties are typically considered perpendicular to the plane of the macrocycle and there-fore are not in conjugation with the porphyrin system. One design strategy for increasing the light absorption of porphyrin sensitizers is to lock the meso-phenyl ring into the plane of the macrocycle in order to improve the

-conjugation between the porphyrin and the phenyl rings. This strategy has the added benefit of increas-ing the molecular asymmetry of the molecule [122]. The combination of added -conjugation and increased molecular asymmetry manifests itself as a red shift in the absorption spectrum of the molecule. Imahori and co-workers attempted this strategy with the dye Fused-Zn-1 (Fig. 20). They found that the fused porphyrin absorption was red-shifted and broadened, and it was a more effi-cient sensitizer than the unfused analog ( = 4.1% versus 2.8%) under similar experimental conditions [122].

Although meso-carboxyphenyl tethered porphyrins have been studied a great deal in DSSCs, there remains a concern about the decoupling of the phenyl rings from the conjugated system of the macrocycle, which potentially limits the electronic communication between the excited dye and the linkage to the semiconductor. Recently, more attention has been paid to other structural designs such as porphyrins with meso-alkynylbenzoic acid or -linked conjugated tethers; devices using these designs have reached remarkably high efficiencies.

Porphyrins anchored through meso-alkynylbenzoic acid groups. The use of meso-linked alkynylbenzoic acid tethers is a relatively new design strategy in porphyrin DSSC research. The first report of alkynylbenzoic acid-tethered porphyrins was by Stromberg et al. in 2007 [123] in which the reported photocurrent of the device was only 0.035 mA/cm2. Since then two main groups (Dr. Joseph Hupp and Dr. Eric Diau) have studied this novel tether style and DSSC devices utilizing them. The use of the meso-alkynylbenzoic acid tether has led to an increase in solar cell efficiency for porphyrin DSSCs and even put forth a series of porphyrin dyes that outperformed the standard dye N719 under similar experimental condi-tions [124, 125].

It is worth noting that many DSSC studies not only report their laboratory-measured cell efficiencies ( ), but also cite their measured efficiency of N719 dye as a relative standard of cell efficiency. Although N719 has been reported in cells operating at up to 11% efficiency [126], the variability of cell preparation in different labs often makes absolute efficiency comparisons difficult to compare (and very few labs can get N719 to operate at efficiencies that high). When absolute cell efficiencies are required, the National Renewable Energy Laboratory provides a certified measurement service [127].

In papers by Hupp and co-workers, dyes 1b and 3 (Fig. 21) were reported to give efficiencies of 2.5 and 1.6% respectively in standard TiO2 DSSCs [128, 129]. In the case of 1b, a comparison with N719 was made and 1b performed at ~71% the efficiency of N719. In the report on 1b, ZnO was also tested as a semiconductor, and it was determined that the more acidic dyes that tend to slightly corrode the ZnO surface showed better elec-tron injection dynamics. In the case of dye 3, the use of

N

N

N

N Zn

COOHFused-Zn-14.1%

Fig. 20. A fused naphthalene porphyrin [122]

N

NH

NHN

CO2H

CO2H

HO2C

HO2C TCPP = 3.5%

N

N

N

N Zn

CO2Hmono-ZnP = 6.5%

N

Fig. 19. Chemical structures of the most efficient free-base [105] and zinc-metallated [121] meso-substituted porphyrins having a benzoic acid linking group

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anchoring groups was first demonstrated in 2004 by Nazeeruddin et al. when they introduced

a dye with an energy conversion efficiency of 4.1%.155 Since then β-substituted porphyrins

have achieved efficiencies greater than 7% and have been synthesized to study substituent

effects.154, 156

Although it was proved that modification of the β-position of porphyrin is an effective

strategy to optimize the energy levels and improve the efficiency of DSSCs, porphyrin dyes

have shown lower photovoltage compared with Ru sensitizers.157 One major cause may be

due to a significantly reduced electron lifetime in porphyrin-based DSSCs. In related DSSCs,

the zinc porphyrins gave a better performance in comparison with the corresponding free-base

dyads.158 Probably because zinc-porphyrins have longer S2 excited state lifetimes that is

favorable for hot-injection from higher energy excited states.27 Also, the presence of metal

ions in the porphyrin center effectively changes the macrocycle photophysical and

photochemical properties, which favors the efficiency for charge separation at the interface

and the overall cell efficiency. In 2014, Grätzel et al. have reported a new record on DSSCs

efficiency of 13% after using molecular engineering to tune the desired sensitizer’s properties

on a zinc-porphyrin.1

CHAPTER 3: Electrostatic Adsorption of H2TPyP[Ru(bpy)2NO2]4 on mesoporous TiO2 Thin Films.

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CHAPTER 3: Adsorption of a Free-Base

Porphyrin meso-substituted with Ruthenium(II)

groups, H2TPyP[Ru(bpy)2NO2]4, on Mesoporous

TiO2 Thin Films, in the Absence of Functional

Binding Groups.

3.1. INTRODUCTION

The work done during the first 2 years of the current DSc proposal was basically

centered on the photophysical properties and characterization of porphyrin compounds. A

wide variety of techniques were employed to accomplish such task, for instance steady state

spectroscopy (UV-vis absorption and photoluminescence), time-resolved spectroscopy

(excited state lifetime and transient absorption) and excited state non-linearity. In 2013 the

opportunity to join in the governmental program Science without Borders (Ciência sem

Fronteiras – CsF) as an exchange visiting student in the USA was fulfilled in an outstanding

research laboratory led by Prof. Gerald J. Meyer, in the Johns Hopkins University (JHU),

Baltimore – MD, USA. In addition to improve the knowledge in the photophysical and

photochemical research field as well as the learning of new experimental techniques, the

major focus of research at Meyer’s groups covers one of the world most highlighted research

topics: the solar energy conversion to energy – the solar cells – and more specifically, the dye

sensitized solar cells (DSSC), considered to be the potential candidates for the future of solar

energy conversion.

In this fashion, the research work line proposed as a visiting student at JHU would

involve porphyrins obtained from collaborative networks of Prof. Newton Barbosa Neto with

a synthesis specialized group from Prof. Alzir Batista in the Departamento de Química,

Universidade Federal de São Carlos (UFScar), São Carlos – SP, Brazil. The said collaboration

was established in the past given the seriousness that Prof. Newton leads his research. A


82

group of porphyrin ruthenium-base complexes were selected to the following work in

Meyer’s laboratories with the possibility to employ them as sensitizers in DSSC.

The current PART II of this dissertation, composed by Chapter 3, tries to create a

coherent “bridge” between what was done prior to the experience at JHU, and what ended up

being the convergence to a possible new research line adopted by Prof. Newton group – the

observation of photodriven charge and energy transfer in organic and inorganic system. While

PART I remains focused in the analysis of porphyrin systems, PART III is completely

devoted to phenomena of processes enclosed in DSSC systems. The sudden deviation will be

satisfactorily discussed in this chapter.

The current section of this dissertation is focused on a new free base tetrapyridyl

porphryin meso-substituted with a ruthenium complex, Ru(dmb)2NO2, Figure 3.1. The said

molecule, although absent of functional binding groups, was suitable for sensitization of TiO2

through what is believed to be electrostatic interactions with the nanocrystallite surface. The

porphyrin extensively discussed in Chapter 2 has also been tested for TiO2 sensitization, but

the attachment was very inefficient, certainly because of the absence of binding groups.

Figure 3.1. Molecular structures of {H2TPyP[Ru(dmb)2NO2]4}(PF6)4, where R = [Ru(dmb)2NO2]PF6.

Here (dmb) stands for 4,4′-Dimethyl-2,2′-dipyridyl.

For reasons discussed above, the only attempt to studied porphyrin complexes

adsorbed onto TiO2 surfaces was done for the H2TPyP-RuNO2.


83

3.2. EXPERIMENTAL METHODS

3.2.1 – Materials and Preparations

Materials. The following reagents were used as received from the indicated commercial

suppliers: acetonitrile (CH3CN; Burdick & Jackson, spectrophotometric grade); lithium

perchlorate (LiClO4; Sigma-Aldrich 99.99%); Rhodamine B (Sigma Aldrich, ≥95.0%); argon

gas (Airgas, >99.998%); and glass microscope slides (Fisher Scientific, 1mm thick).

Preparation. The ruthenium and porphyrin compounds were synthesized by the group of Prof.

Alzir Batista from the Departamento de Química of the Universidade Federal de São Carlos.

Transparent TiO2 nanocrystallites (anatase, ~15 nm in diameter) were prepared by

acid hydrolysis of Ti(i-OPr)4 using a sol-gel method previously described in the literature.143

The sols were cast as transparent mesoporous thin films by doctor blading onto glass

microscope slides for spectroscopic measurements and transparent FTO conductive substrates

for electrochemical measurements with the aid of transparent cellophane tape as a mask and

spacer (~10 µm thick). The films were sintered at 450 ºC for 30 minutes under an atmosphere

of O2 flow and either used immediately or stored in an oven for future use. Sensitization was

achieved by immersing the thin films in acetonitrile sensitizer solutions (mM concentrations)

for hours to days depending on the desired surface coverage.

3.2.2 – Spectroscopy

UV-Vis Absorption. Steady-state UV-visible absorption spectra were obtained on a Varian

Cary 50 spectrophotometer at room temperature in 1.0 cm path length quartz cuvette. The

solutions were prepared with known concentration, from which was obtained their extinction

coefficient, 𝜀, using Beer-Lambert’s Law. For further studies of photoluminescence and

transient absorption the solutions were purged with argon gas for a minimum of 20 minutes.

Sensitized TiO2 thin films were positioned at a 45º angle in cuvettes filled with the indicated

acetonitrile solutions. The solutions were purged with argon gas for a minimum of 30 min

prior to transient absorption and spectroelectrochemical studies.


84

Photoluminescence. Steady-state photoluminescence (PL) spectra and excitation spectra (ES)

were obtained with a Spex Fluorolog spectrophotometer equipped with a 450 W Xe lamp for

the excitation source, with samples dissolved in acetonitrile, argon gas purged, and placed in a

1.0 cm thick quartz cell. Was used a 90o configuration for detection of the signal. The

excitation wavelengths were adjusted using a monochromator placed between the light source

and the sample. The concentration employed in these optical measurements were controlled

via absorption measurements such that the absorption at the excitation wavelength did not

exceed 0.2, in order to avoid reabsorption effects.

Photoluminescence quantum yield was calculated using the actinometry method where

Rhodamine B was the reference standard.

Transient absorption. Nanosecond transient absorption measurements were obtained with an

apparatus similar to that which has been previously described in the literature.82 Briefly,

samples were excited by a Q-switched, pulsed Nd:YAG laser (Quantel U.S.A. (BigSky)

Brilliant B; 5−6 ns full width at half-maximum (fwhm), 1 Hz, ∼10 mm in diameter) tuned to

532 nm with the appropriate nonlinear optics. The excitation energy was measured by a

thermopile power meter (Molectron) and was typically 1−5 mJ/pulse so that the absorbed

energy was typically <1 mJ/pulse. A 150 W xenon arc lamp served as the probe beam and

was aligned orthogonal to the laser excitation light. The lamp was pulsed with 100 V for

detection at sub-100 µs time scales. Detection was achieved with a monochromator (Spex

1702/04) optically coupled to an R928 photomultiplier tube (Hamamatsu). Appropriate glass

filters were positioned between the probe lamp/sample and the sample/detection

monochromator. Transient data was acquired with a computer-interfaced digital oscilloscope

(LeCroy 9450, Dual 350 MHz) with an overall instrument response time of ∼10 ns. Typically,

30 laser pulses were averaged at each observation wavelength over the range 400−750 nm, at

3 or 5 nm intervals. Full spectra were generated by averaging 2−10 points on either side of the

desired time value to reduce noise in the raw data. For single wavelength measurements,

90−180 laser pulses were typically averaged to achieve satisfactory signal-to-noise ratios.


85

3.3. RESULTS AND DISCUSSIONS

3.3.1 – Steady State Experiments and the Electrostatic Interaction with

TiO2.

The electronic spectra of the porphyrin H2TPyP-RuNO2 and the adsorbed H2TPyP-

RuNO2/TiO2 are present in Figure 3.2A. The H2TPyP-RuNO2 UV-Vis absorption spectrum

presented strong peaks at the UV spectral region, where the well-known singlet high energy

transition, the Soret band was found at 417 nm, with ε = 162,000 M-1 cm-1. Regarding to the Q

bands, the characteristic D2h symmetry of the porphyrin reflects their splitted four-banded

absorption spectrum in the Q-band region,84 localized at 516 nm, 552 nm, 590 nm and 646

nm.

Figure 3.2. (A) Steady-state UV-Vis absorbance spectra of H2TPyP-RuNO2 in acetonitrile solution

(black) and the electrostatically attached compound to the TiO2 surface, H2TPyP-RuNO2/TiO2 (red).

The black dotted line separates the absorption range from the dye to the fundamental absorbance of the

TiO2. (B) Normalized steady state excitation spectrum, probed at 650 nm, (black) and

photoluminescence spectrum, excited at 532 nm, (red) of H2TPyP-RuNO2 in acetonitrile solution.

Similarly to what was observed in Chapter 2, when compared to the H2TPyP, the

H2TPyP-RuNO2 shows intense absorption band in the ultraviolet spectral range ascribed to

intraligand π → π* transitions within the (dmb) ligands. The activation of new vibrational

modes after the complexation of H2TPyP with the Ru(dmb)2NO2 triggered the broadening and


86

the decrease in oscillator strength of the Soret band. The ruthenium groups did not

significantly perturbed the porphyrin lowest singlet excited state, S1, maintaining the phyllo

type for the Q bands relative intensity, which is supported by the excitation spectrum, Figure

3.2B. The metal-to-ligand charge transfer (MLCT) of the Ru(dmb)2NO2 is overlapped with

the porphyrin Soret band. Sensitization achieved through electrostatically adsorbing the

porphyrin dyes on the TiO2 surface led to negligible changes in the UV-vis absorption

spectral shape, Figure 3.2A.

PL experiments on H2TPyP-RuNO2 exhibited very similar photoluminescence spectral

features with the unsubstituted H2TPyP,13 with peaks at 650 nm and 711 nm.

Photoluminescence quantum yield calculation found 0.06%, value expressing efficient excited

state quenching mechanisms through non-radiative relaxation processes back to the ground

state.

The conclusions proposed in Chapter 2 are very similar to what was observed in

preliminary spectroscopic studies on the H2TPyP-RuNO2 porphryin.

3.3.2 – Transient Absorption and the Possibility of Electron Injection.

Solution transient absorption experiments on H2TPyP-RuNO2, probed at 470 nm, have

revealed the existence of an exponential relaxation process, yielding lifetimes of 130 µs

(40%) and 12 µs (60%), Figure 3.3A. Porphyrin with peripheral ruthenium groups have

shown to be very sensitive in their triplet excited states. Usually, the observed trend is the

porphyrin-ruthenium compound having a much larger triplet excited state life time when

compared to the porphyrin without the ruthenium groups (0.6 µs). This behavior was better

observed and discussed in Chapter 2.13-15, 71, 75 Although there exist two different lifetimes to

the transient state disappearance, it still remain unknown the real source to address such

processes.

Once was obtained a high surface coverage for H2TPyP-RuNO2 sensitized TiO2 thin

films, transient absorption experiments were found to be suitable to investigate whether

charge injection could appear in such system. The transient absorption spectrum of a free base

tetrapyridyl porphyrin shows a characteristic bleach of the ground state between 400 nm and

430 nm, and a growth from 430 nm to 500 nm. The results showed clear similarity in the

transient absorption spectral shape of H2TPyP. Besides the Soret band bleach, other small


87

bleaches to longer wavelengths of the transient absorption spectrum revealed the structuration

of the Q bands. The pronounced growth starting at 650 nm could be immediately

misunderstood as TiO2(e-) in the conduction band, if the certainty of electron injection is not

proven to be the case. Moreover when adsorbed on the TiO2 surface, the transient kinetic trace

of H2TPyP-RuNO2, did not accept bi-exponential adjustments. Instead, the adjustment to the

KWW fitting function combined with a bi-exponential function revealed the presence of a fast

deactivation process.

Figure 3.3. Single wavelength kinetics (470 nm) transient absorption of H2TPyP-RuNO2: (A) in

acetonitrile solution; and (B) adsorbed on the TiO2 surface immersed in neat acetonitrile.

Figure 3.4. (A) Transient absorption spectra after pulsed 532 nm excitation of H2TPyP-RuNO2/TiO2

in neat acetonitrile. (B) Its normalized representation. The inset figures show the rescaled transient

spectra from 500 nm to 750 nm for better visualization.


88

The found fitting parameters are: 270 ns (44%), 10 µs (33%), and 70 µs (23%). Many

porphyrins used in DSSCs are known to have lower cell efficiencies due to fast electron

recombination processes, so that the observed fast component responsible for 44% of the ∆𝐴

signal could be associated to the back electron transfer from the TiO2 to the oxidized

porphyrin. Although speculative, such results reinforce the necessity to perform photoelectric

experiments to make sure whether current can be generated by the sensitization of H2TPyP-

RuNO2 in TiO2 thin films.

The potential energy diagram for electron transfer process showed a favorable driving

force for electron transfer from the ruthenium peripheral groups to the porphyrin ring, Figure

3.5. Although transient absorption experiments are able to address many electron transfer

processes, in this case would be necessary to perform spectroelectrochemical experiments to

better understand the oxidized and reduced forms of the absorbance spectrum of the porphyrin

compound. So that by comparison with the photogenerated transient absorption spectrum, it

would be possible to satisfactorily address specifics changes in absorbance spectrum to

electron transfer processes.

Figure 3.5. Reduction potential diagram for electron transfer processes of H2TPyP- RuNO2. The black

lines are potentials associated with the porphyrin ring while the red lines are the potentials for the

RuNO2 moiety, in acetonitrile.


89

3.4. CONCLUSIONS

The attempts to sensitize TiO2 nanoparticle with porphyrins for charge transfer studies

were found to be unpromising. Chapter 2 gives an overall possible explanation for the

unsatisfactory mechanism. In brief, complexation of substituted free base porphyrin with

peripheral ruthenium groups in the porphyrin ring meso-position mainly interact with higher

energy excited states from the π-conjugated system of porphyrins which are known to have

very fast lifetime (~fs), while the long lived S1 states do not overlap with the ruthenium

complex orbital for possible intermolecular charge transfer. Also, the absence of functional

binding groups does not create stable adsorption of the porphyrin on the semiconductor

surface, moreover the presence binding groups, such as the carboxylic acid (COOH), gives

the moiety an electron withdrawing characteristic which is favorable to charge transfer.

Photoelectron injection could be present in transient absorption experiment, however to truly

precise this information, photoelectric experiments (e.g. incident photon-to-current efficiency

– IPCE) should be performed.

The initially proposed work plan of porphyrin studies as possible dye sensitizers for

DSSCs was found to be worthless due its strong limitations. However very good insights and

finding were of fundamental importance for a better understand on the working mechanism of

DSSC and the necessity of molecular engineering to tune porphyrin photophysical and

photochemical properties.

With the failure of the initial proposal, alternative works had to be initiated. The fact

that Prof. Meyer’s laboratory was specialized in ruthenium polypyridyl complexes, and not

such proximity with porphyrin synthesis, was the first step in the research line changing. The

fundamental purpose when working in Prof. Meyer’s laboratory was to learn the overall

science that rules the working mechanism of the DSSCs. In this fashion, adjust the

possibilities to the needs was an inherent act.

The research focused on the ruthenium-based dyes started through the understanding

of the photophysics and photochemistry of such inorganic coordination groups. Latter, I was

introduced to a very interesting effect found in DSSCs that happens right after electron

injection, called the Stark effect. In summary, such effect is the perturbation of the

photophysical and photochemical properties of the anchored dyes caused by the presence of a

surface electric field originated from the photoinduced injected electrons. The Stark effect in


90

DSSCs has recently being a target of exaustly research as the presence of a surface electric

field might be relevant to the overall working mechanism of the dye sensitized solar cells.

91

PART III: Stark Effect in Mesoporous TiO2 Thin Films Sensitized With

Pyridine Based Ruthenium Compounds.

INTRODUCTION – PART III

92

INTRODUCTION

An Introduction to the Stark Effect and its Implications to the Dye

Sensitized Solar Cells

Johannes Stark observed splittings of the hydrogen atom induced by an electric field

nearly 100 years ago,159-160 and his name has been associated with this class of effects ever

since. Measurements of Stark splittings of spectral lines in the gas phase continue to this day,

both to obtain fundamental properties of states and transitions and as a diagnostic tool in

complex systems such as plasmas. Stark spectroscopy is less widely applied in condensed

phases, as large electric fields are needed to detect the effect for inhomogeneously broadened

lines.160-161

In a molecular level understanding, an external electric field applied to a molecule will

cause a reorganization of its electron distribution, resulting in a change in the Hellmann-

Feynman force acting on the constituent nuclei. The field also exerts a force directly on the

nuclei. The resulting net force will induce changes in molecular geometry, vibrational and

electronic properties.162

The evolution of the Stark effect concept and its potential use as a tool to understand

molecular properties has been attractive, for many years, to the research community. A

particular research area, created ever since, of attractive interesting and remarkable

importance is the application of the Stark effect in spectroscopy to experimentally determine

the change in dipole moment, ∆µ, and polarizability, ∆α, for a transition in molecular systems.

This research area was then named the Stark Spectroscopy.161, 163

In the conventional Stark spectroscopy, application of an external electric field to

molecules that are immobilized and not oriented, e.g. in a glass or polymer film, leads to the

broadening, shifts and change in intensity of the electronic absorption spectra, due to changes

in dipole moment, ∆µ, polarizability, ∆α, and oscillator strength, respectively. The

contributions of each type of electro-optic property can be extracted by comparing the

observed change in absorption in the field with the second, first and zeroth derivatives of the

absorption spectrum, respectively. For an isolated absorption band in a non-oriented,

immobilized sample where the shift of the transition energy upon application of an electric


93

field 𝐸, ∆ν (usually expressed in units of cm-1), is smaller than the inhomogeneous bandwidth,

the change in absorption upon application of an external field is given by: 161, 163

Δ𝐴 𝜈 = 𝐸! 𝐴!𝐴 𝜈 + !!!"#!

𝜈 !!"

!(!)!

+ !!!"!!!!

𝜈 !!

!!!!(!)!

(III-1)

A better description of Equation III-1 can be found in references [161] and [163]. But

briefly, decomposition of the ∆A spectrum for an isotropic immobilized sample into

absorption derivative components yields values for 𝐴!, 𝐵! and 𝐶! which then can be used to

extract information on the molecular parameters: transition moment (𝑚), polarizability (∆α)

and change in dipole moment (∆𝜇).

In the simplest physical terms, Equation III-1 can be better understood as following.

The random orientations of ∆𝜇 relative to the applied field direction lead to a spread in the

transition energy; the difference between this broadened lineshape and the lineshape in the

absence of a field is just the second derivative of the absorption. The molecular polarizability

change, ∆α, interacts with the field to induce a dipole moment typically in the direction of the

field. Because this field-induced dipole moment is oriented with respect to the field, the

spectrum shifts either to higher or to lower energy, depending on the sign of ∆α; this

lineshape is the first derivative of the absorption spectrum.163

Many molecules, especially those asymmetric characterized by electron donating and

withdrawing groups, exhibit strong dipole moments in both ground and excited state, as

consequence of the distribution of electron density across the molecule. Dipole moments are

defined to point from the negative charge (δ−) to the positive charge (δ+). Some molecules

may have, in their ground state, an intrinsic distribution of charges which leads to the

appearance of a ground state dipole moment, (𝜇!). When In their excited state, molecules

usually undergo charge transfer processes between its molecular orbitals generating a new

charge density configuration, leading to a dipole moment in the excited state (𝜇!"). The

preferential directions of each dipole moment, (𝜇!) or (𝜇!"), is highly dependent on the

properties of the molecules with their electron donor-acceptor moieties interactions. The

change in dipole moment upon excitation (∆𝜇) is defined as:

∆𝜇 = 𝜇!" − 𝜇! (III-2)

This quantity gives information about the magnitude of the intramolecular charge


94

transfer upon excitation and is therefore an important parameter in the characterization dye

molecules, especially for Stark effect experiments.

An electric field applied to a molecule will cause a reorganization of the electron

distribution, such that the resulting net force is responsible for noticeable induced effects:

changes in the intensity of electronic and vibrational transitions bands, shifts in their

frequencies and degeneracy splitting.164

In very general terms the change in transition energy (∆𝑈) due to an external electric

field (𝐸) is given by,

∆𝑈 = −Δ𝜇 ∙ 𝐸 − !!𝐸 ∙ ∆𝛼 ∙ 𝐸 (III-3)

where is evident the presence of a first order Stark effect, which is linear in the electric field,

and the second order Stark effect , which is quadratic in the electric field. Cappel et al have

extensively worked with the Stark spectroscopy on organic dyes used in dye sensitized solar

cells and they found that changes in the transition absorption spectrum were linear with the

electric field,165 so that Equation (III-3) can be reduced to ∆𝑈 = −Δ𝜇 ∙ 𝐸. Also in this work,

Cappel et al have proposed that the contribution of the Stark Effect, under the limitation to

small perturbation approximation, could be well modeled by the first derivate of the ground-

state spectrum, given by a relation that accounts for the spectral shift of wavenumbers Δ𝜈, in

cm-1, of the transition related to a change in dipole moment, ∆𝜇, under the presence of an

electric field 𝐸.165-166

∆𝐴 = !"!!Δ𝜈 = !"

!"!∆!∙!!!

(III-4)

Considering dye-sensitized solar cells, there is a crescent search for understandings of

how the Stark effect affects electronic transitions of dye molecules adsorbed to titanium

dioxide surfaces. In this case, the perturbation of an electric field is originated from electrons

that were injected, from a molecular excited state, into the TiO2 nanoparticles right after light

absorption. As the dye molecules are adsorbed onto the nanoparticle surface and the electric

field is emanated from its surface, it is convenient to define a main direction of the electric

field, which is normal to the titanium dioxide surface. On average, dye molecules adsorbed to

the surface should therefore experience an almost uniform electric field in one direction, so


95

that the relation ∆𝑈 = −Δ𝜇 ∙ 𝐸 rewritten in a one-dimensional form (which can be chosen to

be along the normal vector to the TiO2 nanoparticle):

∆𝑈 = −Δ𝜇 ∙ 𝐸 (III-5)

Figure III-1. Electric field direction outside the TiO2 nanoparticle in which point charges are

presented inside the TiO2. On the right is shown the first order Stark shift in both cases, when 𝚫𝝁 is

parallel and antiparallel to the electric field 𝑬. The gaussians in black represent the initial conditions of

a random absorption transition while those in red are the perturbed transitions by an external electric

field resulting in frequency shifts. The quantity ∆𝑨 is the difference spectrum ∆𝑨 = 𝑨𝒇𝒊𝒏𝒂𝒍 −

𝑨𝒊𝒏𝒊𝒕𝒊𝒂𝒍.

The scheme shown in Figure III-1, summarizes the possible alignments configurations

between the change in dipole moment and the surface electric field leading to the resultant

frequency shift of the molecular transitions.


96

Traditional electroabsorption analysis, through the classical Stark Spectroscopy,

predicts that a change in dipole moment, ∆𝜇, would give rise to a second-derivative spectrum

rather than the first-derivative. 161, 163, 167-168 When electron are injected into the TiO2

nanoparticles, its electric field is emanated preferentially normal to the TiO2 surface, while the

overall orientation of sensitizers remains unchanged. This makes the field orientation with

respect to each sensitizer’s lowest energy change in dipole moment always predominantly in a

collinear configuration, resulting in an unidirectional shift of the absorption spectrum, which

resembles its first derivative spectrum.169

Most of the Transient Absorption employed in the study of dye sensitized solar cells

end up on the following equation:

∆𝐴 = ∆𝐴!"#$#%&$ + ∆𝐴!" + ∆𝐴!"!#$%&'( + ∆𝐴!"#$% (III-6)

where ∆𝐴 express the difference in absorbance between an initial state, where molecules are

presented in its ground state, and a final state which might be composed by oxidized

molecules, excited state, the presence of electrons in the TiO2 conduction band, and the

shifted ground state absorbance, so that it generates ∆𝐴!"#$#%&$ , ∆𝐴!" , ∆𝐴!"!#$%&'( , and

∆𝐴!"#$%, respectively.

The injected electron in TiO2 nanoparticle leaves behind an oxidized dye, which is

regenerated on the nanosecond time scale by the redox chemistry of I-/I3-. The excited state

for the molecule under study has its life time around 300 nanoseconds in neat acetonitrile and

is substantially quenched by the addition of cations in the electrolyte. With this in mind,

within a secure time window starting after the first 1µs, the Equation III-6 can be reduced to:

∆𝐴 = ∆𝐴!"!#$%&'( + ∆𝐴!"#$% (III-7)

As the electron extinction coefficient at the UV-Vis spectral frame is considerably

small, showing reasonable evidence of its existence only at the red spectral region, the major

observed ∆𝐴 within the UV-Vis spectral frame is specifically attributed to the Stark Effect.

In this dissertation work, the Stark effect was observed in two different types of

measurements: 1) in Spectroelectrochemical experiments in which electrochemically

accumulation of electron in the density of states of the TiO2 nanoparticles is achieved by the

application of negative potentials; and 2) in Transient Absorption Experiments, which the


97

electric field is induced after photoinjection from the excited state dye molecules adsorbed on

the TiO2 surface.

In Chapter 4, is proposed the analysis of a heteroleptic ruthenium complex adsorbed

on TiO2 thin films that have its absorption spectrum perturbed by electric fields from either

electrochemically accumulation of electrons or injected electrons right after pulsed laser

excitation. The Stark effect is systematically studied as a function of four different types of

cations adsorbed on the TiO2 surface: Na+, Li+, Mg2+ and Ca2+. The nature of each cations

critically defines the strength of the electric field ant the TiO2 – dye molecule interface based

on screening mechanisms.

While in Chapter 4, the Stark effect is observed in the presence of metal ions salts of

perchlorates and the redox mediator I-/I3-, Chapter 5 gives an approach in which a ruthenium

compound composed by a mono-pyridine ligand is absorbed to the TiO2 surface. The

ruthenium complex showed efficient electron injection in the absence of adsorbed cation and

excited state formation wasn't observed under the experimental conditions used. Under such

conditions, the observed ∆𝐴 in transient spectroscopy was composed exclusively by

∆𝐴 = ∆𝐴!"#$#%&$ + ∆𝐴!"!#$%&'( + ∆𝐴!"#$%. It was possible to probe the Stark effect in neat

solvent, acetonitrile, in transient absorption experiments. Critical insights, which concerns

about charge motion and screening the high TiO2 permittivity, were satisfactorily obtained.

Finally, Chapter 5 proposes a new model to report the time evolution of the Stark effect,

under restrict conditions.

CHAPTER 4 – Electric Fields and Charge Screening in Dye Sensitized Mesoporous Nanocrystalline TiO2 Thin

Films.

98

CHAPTER 4 – Electric Fields and Charge

Screening in Dye Sensitized Mesoporous

Nanocrystalline TiO2 Thin Films.

4.1. INTRODUCTION

The 1991 Nature paper by Grätzel and O’Regan introduced the clever idea of utilizing

mesoporous thin films of nanocrystalline TiO2 in photoelectrochemical cells170. The idea

turned out to be revolutionary and created whole new fields of science based on energy

conversion with nanometer-sized semiconductor materials.171-172 The nanometer length scale

can result in very different photoelectrochemical behavior than that observed in bulk

semiconductor materials. For example, in single crystal and thin films materials, electron-

hole pairs are efficiently separated by a surface electric field (the ‘depletion’ or ‘space charge’

region) that is absent in weakly doped semiconductor nanocrystallites.173 In fact, the three

bias conditions identified for single crystal semiconductor materials, i.e. depletion, inversion,

and accumulation, are not particularly useful for quantifying or modeling the

photoelectrochemistry of semiconductor nanocrystallites. In the case of dye-sensitized solar

cells (DSSCs), it has generally been assumed that any electric fields that might be present

under illumination would be completely screened from the surface anchored dye molecules by

the large dielectric constant of TiO2, εr = 7 – 50;174 the high permittivity of acetonitrile, εr =

37;175 and the half molar ionic strength of the electrolyte.

The assumption of quantitative charge screening at sensitized TiO2 interfaces was

proven to be incorrect in 2010, when two groups reported that electrons injected into TiO2 had

a profound influence on the absorption spectrum of dye molecules anchored to the surface.165,

176 The absorption changes measured after the injection of charge were similar to those

previously reported in traditional Stark spectroscopic measurements,167-168, 177-178 but were

unidirectional due to the fixed orientation of the molecular dipole moment relative to the TiO2

surface. With some assumptions, the spectral shifts reported directly on the magnitude of the

electric field that was found to be substantial, on the order of 2.7 MV/cm.176 It remains


Films.

99

unclear whether this electric field or charge screening is relevant to power conversion in

DSSCs. The spectral shift of the dye molecules is generally small and does not appreciably

change the light harvesting efficiency of the sensitized thin film. Likewise, the potential drop

experienced by the dye molecules represents a fairly small value of ~ 40 mV that corresponds

to only about 5% of the open circuit photovoltages reported for gold standard DSSCs.176 In

fact, charge screening may have a deleterious influence on energy conversion efficiencies

with anionic redox mediators like I-/I3-. Synonymous to increasing the width of the space-

charge layer in bulk semiconductor solar cells, increasing the Debye length for charge

screening at the semiconductor electrolyte interface should aid in the generation of even

further spatially separated and longer-lived anionic charges, i.e. TiO2(e-)s and I3-, and hence

improve solar conversion efficiencies. While speculative, fundamental studies of surface

electric fields and charge screening may provide new insights into the fabrication of superior

DSSCs. Such studies are also of intellectual interest in their own right.

Figure 4.1. The structure of Ru(dtb)2(dcb)2+.

An intriguing observation from previous research was that following pulsed laser

excitation of sensitized TiO2 thin films immersed in an acetonitrile electrolyte, the magnitude

of the Stark effect decreased over time periods in which the TiO2(e-) concentration was

constant.176 This behavior was attributed to the reorganization of interfacial ions and solvent

molecules responding to the electrons that were photoinjected into TiO2, a process often

referred to as “screening”. While screening of this type is well known in the electrochemical

and photo-electrochemical literature,179-184 to our knowledge this represents the first

opportunity to probe the dynamics of this process on short time scales. The kinetics for

charge screening were reported to be sensitive to whether Mg2+ or Li+ cations were present in

the electrolyte.185 The study of [Ru(dtb)2(dcb)](PF6)2, where dtb is 4,4′-(tert-butyl)2-2,2′-


Films.

100

bipyridine and dcb is 4,4′-(CO2H)2-2,2′-bipyridine, was utilized as the compound is a

particularly sensitive probe of the surface electric field Figure 4.1. Here these studies were

expanded to include Na+ and Ca2+. The inclusion of these ions influenced the screening

dynamics as well as the interfacial density of states and the excited state injection yield.

4.2. EXPERIMENTAL METHODS

4.2.1 – Materials and Preparations

Materials. The following reagents and substrates were used as received from the indicated

commercial suppliers: acetonitrile (CH3CN; Burdick & Jackson, spectrophotometric grade);

deionized water; lithium perchlorate (LiClO4; Sigma-Aldrich 99.99%); sodium perchlorate

(NaClO4; Sigma-Aldrich, 99%); magnesium perchlorate (Mg(ClO4)2; Sigma-Aldrich, ACS

Reagent); calcium perchlorate tetrahydrate (Ca(ClO4)2·4H2O; Sigma-Aldrich, 99%); tetra-n-

butylammonium perchlorate (TBAClO4; Aldrich, ≥99.0%); tetra-n-butylammonium iodide

(TBAI; Fluka, ≥99.0%); argon gas (Airgas, >99.998%); oxygen gas (Airgas, industrial grade);

titanium(IV) isopropoxide (Sigma-Aldrich, 97%); fluorine-doped SnO2-coated glass (FTO;

Hartford Glass Co., Inc., 2.3 mm thick, 15Ω/□); and glass microscope slides (Fisher

Scientific, 1mm thick). [Ru(dtb)2(dcb)](PF6)2 was available from previous studies.176

Preparations. Transparent TiO2 nanocrystallites (anatase, ~15 nm in diameter) were prepared

by acid hydrolysis of Ti(i-OPr)4 using a sol-gel method previously described in the

literature.143 The sols were cast as transparent mesoporous thin films by doctor blading onto

glass microscope slides for spectroscopic measurements and transparent FTO conductive

substrates for electrochemical measurements with the aid of transparent cellophane tape as a

mask and spacer (~10 µm thick). The films were sintered at 450 ºC for 30 minutes under an

atmosphere of O2 flow and either used immediately or stored in an oven for future use.

Sensitization was achieved by immersing the thin films in acetonitrile sensitizer solutions

(mM concentrations) for hours to days depending on the desired surface coverage. Unless

otherwise noted, the thin films were sensitized to roughly maximum surface coverage, Γ ~ 7 x

10-8 mol/cm2, which was determined using a modified Beer—Lambert law,186


Films.

101

Abs = 1000 x Γ x ε (4.1)

where ε is the molar decadic extinction (absorption) coefficient (16,400 M-1cm-1 at 465 nm)

that was assumed to have the same value when anchored to the surface. Sensitized films were

soaked in neat acetonitrile for at least one hour prior to experimentation.

4.2.2 – Spectroscopy and Electrochemistry

UV-Visible Absorption. Steady-state UV-Visible absorption spectra were obtained on a Varian

Cary 50 or an Agilent Cary 60 spectrophotometer at room temperature in 1.0 cm path length

quartz cuvettes. Sensitized TiO2 thin films were positioned at a 45º angle in cuvettes filled

with the indicated acetonitrile solutions. The solutions were purged with argon gas for a

minimum of 30 min prior to transient absorption and spectroelectrochemical studies.

Photoluminescence. Steady-state photoluminescence (PL) spectra were obtained with a Spex

Fluorolog spectrophotometer equipped with a 450 W Xe lamp for the excitation source. PL

spectra of sensitized thin films were obtained under ambient conditions at room temperature

with excitation 45º to the surface and detection from the front face of the sample. Quenching

experiments were performed by obtaining the PL spectrum of the sensitized thin film in neat

solvent and after replacement of the neat solvent with the electrolyte solution of interest.

Transient Absorption. Nanosecond transient absorption measurements were obtained with an

apparatus similar to that which has been previously described in the literature.82 Briefly,

samples were excited by a Q-switched, pulsed Nd:YAG laser (Quantel USA (BigSky)

Brilliant B; 5-6 ns full width at half-maximum (fwhm), 1 Hz, ~10 mm in diameter) tuned to

532 nm with the appropriate nonlinear optics. The excitation fluence was measured by a

thermopile power meter (Molectron) and was typically 1-5 mJ/pulse so that the absorbed

fluence was typically <1 mJ/pulse. A 150 W xenon arc lamp served as the probe beam and

was aligned orthogonal to the laser excitation light. The lamp was pulsed with 100 V for

detection at sub-100 microsecond time scales. Detection was achieved with a monochromator

(Spex 1702/04) optically coupled to an R928 photomultiplier tube (Hamamatsu). Appropriate

glass filters were positioned between the probe lamp/sample and the sample/detection

monochromator. Transient data was acquired with a computer-interfaced digital oscilloscope

(LeCroy 9450, Dual 350 MHz) with an overall instrument response time of ~10 ns. Typically,


Films.

102

30 laser pulses were averaged at each observation wavelength over the range 400 – 750 nm, at

3 or 5 nm intervals. Full spectra were generated by averaging 2-10 points on either side of the

desired time value to reduce noise in the raw data. For single wavelength measurements, 90-

180 laser pulses were typically averaged to achieve satisfactory signal-to-noise ratios.

Relative excited-state electron injection yields were measured by comparative actinometry on

the nanosecond time scale for samples in different metal cation solutions using lithium as the

reference.142, 187

Electrochemistry. A potentiostat (Bioanalytical Scientific Instruments, Inc. (BAS) model CV-

50W or EC Epsilon electrochemical analyzer) was employed for electrochemical

measurements in a standard three-electrode arrangement with a TiO2 thin film working

electrode, a Pt gauze counter electrode (BAS), and a non-aqueous silver reference electrode

(BAS). The ferrocenium/ferrocene (Fc+/0) half-wave potential was measured both before and

after experiments in a 100 mM TBAClO4/acetonitrile electrolyte that was used as an external

standard to calibrate the reference electrode. All potentials are reported versus the normal

hydrogen electrode (NHE) through the use of a conversion constant of -630 mV from NHE to

Fc+/0 in acetonitrile at 25 ºC.188

Spectroelectrochemistry was conducted via simultaneous application of an applied

potential while monitoring the UV-Vis absorption spectra of TiO2 thin-film electrodes in the

indicated electrolytes. Each applied potential was held for 2-3 min, until the absorbance in the

700-900 nm region became invariant in time. Single-wavelength absorption features plotted

as a function of the applied potential were proportional to the cumulative formation/loss of

states; for the TiO2(e-) absorption features, this was directly related to the cumulative TiO2

density of acceptor states.189

Spectroelectrochemical charge extraction measurements were performed on un-

sensitized TiO2 thin films to obtain the extinction coefficient of the TiO2(e-)s. In these

experiments, the absorbance at 700 nm was recorded as the potential was stepped from +200

mV to increasingly negative values. The charge present in the film was measured

coulometrically after stepping the potential back to the original +200 mV value. 190-192 The

absorption values were corrected for the 45º angle of the thin film in relation to the optical

path. Each charge extraction cycle was repeated 3 times at each applied bias.


Films.

103

4.3. RESULTS AND DISCUSSIONS

Thin films of TiO2 on glass substrates were reacted with Ru(dtb)2(dcb)2+, abbreviated

Ru(dtb)2(dcb)/TiO2, in acetonitrile solutions to a maximum surface coverage, Γ~7 x 10-8 M-1

cm2.176 Representative absorption spectra of Ru(dtb)2(dcb)/TiO2 immersed in neat acetonitrile

and 100 mM perchlorate acetonitrile solutions are shown in Figure 4.2A. The

Ru(dtb)2(dcb)/TiO2 samples exhibit a metal-to-ligand charge transfer (MLCT) absorption

band centered at 465 nm in neat acetonitrile and the fundamental TiO2 absorption below 380

nm. The spectrum measured in neat acetonitrile and in 100 mM TBAClO4 (where TBA is

tetra-n-butylammonium) were within experimental error the same. Replacement of the neat

acetonitrile solvent bath with 100 mM metal perchlorate salt acetonitrile electrolytes resulted

in a bathochromic shift, the magnitude of which was dependent on the cation. The MLCT

absorption shifts to ~480 nm for monovalent cations, Li+ and Na+, and to ~486 nm for

divalent cations, Mg2+ and Ca2+, shown in Figure 4.2A.

Visible light excitation of the MLCT absorption band resulted in room temperature

photoluminescence (PL), shown in Figure 4.2B. In neat acetonitrile, Ru(dtb)2(dcb)/TiO2

exhibited a PL maximum at 665 nm. Upon replacement of the neat solvent with 100 mM

metal perchlorate electrolytes: the PL maximum red-shifted and the PL intensity was

quenched to varying extents dependent on the nature of the cation. The corresponding

quantities are compiled in Table 4.1.

Figure 4.2. Steady-state UV-Vis absorbance (A) and photoluminescence (B) spectra of

Ru(dtb)2(dcb)/TiO2 in neat acetonitrile and in the presence of 100 mM metal perchlorate electrolyte.


Films.

104

Table 4.1. Photophysical and electrochemical properties of Ru(dtb)2(dcb)/TiO2 in 100 mM metal perchlorate acetonitrile solutions.

Cation Absmax (nm)a PLmax (nm) ΔGes (eV)b E0(RuIII/II)

(V vs. NHE) and (α)c

E0(RuIII/II*)

(V vs. NHE)d

neat CH3CN 465 665 2.05 --- ---

Li+ 480 700 1.86 1.46 (1.39) -0.40

Na+ 480 690 1.88 1.43 (1.35) -0.45

Mg2+ 486 710 1.88 1.49 (1.58) -0.39

Ca2+ 486 700 1.90 1.50 (1.74) -0.40

aWavelengths are ±1 nm. bThe free energy stored in the excited state. cThe RuIII/II reduction potential and the ideality factor, α. dThe excited state reduction potential calculated using Equation 3.

Electrochemical reduction of un-sensitized TiO2 thin films resulted in a blue shift of

the fundamental absorption and the appearance of a broad absorption in the visible region.

When measured as difference spectra, the blue shift of the fundamental absorption appears as

a bleach, the magnitude of which was sensitive to the identity of the cation, Figure A4.1 in the

Appendix. Charge extraction experiments were performed on un-sensitized TiO2 thin films to

determine the molar extinction coefficients of the TiO2(e-) absorption band in each of the four

metal perchlorate electrolytes. The absorption at 700 nm was monitored while the applied

potential was stepped from 200 mV to -400 mV vs. NHE for 65 s and then returned to the

initial 200 mV potential, Figure A4.2 in the Appendix. The absorption was corrected for the

45º angle of the thin film relative to the optical path. Plotting the corrected absorbance versus

the extracted charge from the film at multiple potentials allowed for determination of the

molar extinction coefficient from the slope of a linear fit to the data, shown in Figure A4.3 in

the Appendix. The extinction coefficient was independent of the electrolyte composition

within experimental error and was determined to be ε(TiO2(e-)) = 930 ± 50 M-1 cm-1 at 700

nm. The application of more negative potentials, i.e. < -1.2 V, resulted in new absorption

features that were not studied in detail due to the irreversible nature of the absorption changes,

Figure A4.4 in the Appendix.

In order to understand the ground-state behavior, spectroelectrochemistry was

performed on sensitized TiO2 thin films in 100 mM metal perchlorate acetonitrile electrolytes.

Application of a positive applied potential resulted in spectral changes consistent with the

oxidation of RuII to RuIII, shown in Figure A4.5 in the Appendix. The equilibrium potential


Films.

105

where the concentration of RuII and RuIII were equal was taken as the Eº(RuIII/II) reduction

potential for Ru(dtb)2(dcb)/TiO2. The spectroelectrochemical data were fit to Equation 4.2:

𝑥 = !

!!!!!"# !!""!!!

!×!" !"

(4.2)

where x is the fraction of molecules in each oxidation state, Eapp is the applied potential, and α

is the ideality factor. Knowledge of Eº(RuIII/II) allowed for the estimation of the reducing

power of the excited state which was calculated through a free energy cycle using Equation

4.3:

𝐸! Ru!!!/!!∗ = 𝐸! Ru!!!/!! − ∆𝐺!" (4.3)

where ΔGES is the Gibbs free energy stored in the MLCT excited state determined by a

tangent line extrapolation back to zero intensity on the high energy side of the PL spectrum.

The formal reduction potentials and ideality factors are summarized in Table 4.1.

Application of a forward (negative) bias resulted in reduction of TiO2 that was

monitored by the characteristic broad absorption features from 400 to 900 nm attributed to

TiO2(e-)s. Concomitant with the TiO2(e-) absorption, the MLCT absorption band blue-shifted.

Both of these spectral features are evident in Figure 4.3 for a Ru(dtb)2(dcb)/TiO2 thin film in

100 mM LiClO4 acetonitrile solution with an applied bias ranging from 150 to -750 mV vs.

NHE. The normalized spectroelectrochemical absorption spectra are shown in Figure 4.3A

and after subtraction of the contributions from TiO2(e-)s in Figure 4.2B. Difference spectra of

these same data are shown in Figure 4.3C and D.

The electric field experienced by the surface-bound sensitizers as a function of the

applied potential bias was calculated using both peak-to-peak and first-derivative analysis

methods and are compared in the insets of Figure 4.3A and B. For the peak-to-peak analysis,

the electric field was calculated using Equation 4.4:

Δ𝜐 = − !! ∙ ! ∙!"#!!""#!

(4.4)

where h is Planck’s constant, c is the speed of light in a vacuum, Δ𝜐 is the change in

spectroscopic peak maximum (in wavenumbers), Δ𝜇 is the change in dipole moment vector


Films.

106

between the ground and excited state, 𝐸 is the electric field vector, and θ is the angle between

the latter two quantities. With the assumption of θ = 180º and Δ𝜇 = 4.75 D,167, 193 the electric

field can be calculated at each applied bias. Similarly, the first-derivative analysis was

performed using Equation 4.5:

Δ𝐴 = − !"!"

!!!!

(4.5)

where Δ𝐴 is the difference spectrum, or delta absorbance, and !"!"

is the first derivative of the

absorbance spectrum (in wavenumbers).165 The electric field calculated using both the peak-

to-peak analysis and the first-derivative analysis were in good agreement, as seen in the insets

of Figure 4.3A and B.

Figure 4.3. Spectra of a potentiostatically controlled Ru(dtb)2(dcb)/TiO2 film in 100 mM LiClO4

acetonitrile solution (A); and after subtraction of the long-wavelength TiO2(e-) absorption (B). The

difference spectra for the data shown in A and B are given in C and D, respectively. The insets in A

and B indicate the electric field strength calculated by two different analyses. The spectra in dark blue

were recorded at +150 mV and spectra recorded at more negative potentials (up to -750 mV) are

indicated in red. The arrows indicate the direction of change with increased negative applied potential.


Films.

107

The electric field experienced by the surface-bound sensitizers was calculated using

the electron corrected spectra and the first-derivative method for all four cations and is shown

in Figure 4.4 as a function of the applied potential (A) and the estimated electron

concentration per TiO2 nanoparticle (B). The latter was calculated by converting the applied

potential to the number of TiO2(e-)s per 15 nm diameter particle through the measured

absorbance and Beer’s law using the measured extinction coefficient and the effective optical

path length for a 10 µm thick film of 50% porosity.194 For example, in 100 mM metal ion

perchlorate solution where ε = 930 cm-1, an absorbance of 0.0124 would correspond to 20

TiO2(e-)s/particle with Equation 4.6:

𝐴𝑏𝑠 = 𝜀 ×𝑙×𝑐(𝑇𝑖𝑂! 𝑒! =

930 𝑀!!𝑐𝑚!! ×14.14×10!! 𝑐𝑚 ×50%× !"#!(!!)!!! !.!×!"!! !

!×!!×!"! ! !!!

(4.6)

Figure 4.4. Electric field experienced by Ru(dtb)2(dcb)/TiO2 in acetonitrile solutions containing 100

mM Li+, Na+, Mg2+, or Ca2+ as a function of: (A) the applied potential and (B) the number of TiO2(e-)s

on a per particle basis.

The relative injection yields were measured by comparative actinometry 100 ns after

pulsed 532 nm light excitation of Ru(dtb)2(dcb)/TiO2 in 100 mM metal perchlorate

acetonitrile solutions.168 The yields were within experimental error unity for Li+, Mg2+, and

Ca2+ and found to be 0.95 for Na+.

Pulsed light excitation into the MLCT absorption band of Ru(dtb)2(dcb)/TiO2 thin

films immersed in 100 mM metal perchlorate solutions with 250 mM of tetrabutylammonium

iodide, present to regenerate the sensitizer, generated long-lived charge separated states,


Films.

108

comprised of TiO2(e-)s and triiodide. Representative transient absorption spectra shown in

Figure 4.4 were obtained 2.5 µs after laser excitation, a delay time chosen to ensure that all

sensitizers had been regenerated and all iodide oxidation chemistry was complete. The

transient absorption spectra exhibit: (1) small absorption features from 400 – 425 nm,

attributed to formation of triiodide; (2) a first-derivative shaped feature centered around 485

nm, attributed to the TiO2(e-)-induced Stark effect; and (3) absorption from 600 – 750 nm,

attributed to TiO2(e-)s. The transient absorption spectra were modeled with first-derivatives

of the ground-state absorption spectra, shown in Figure A4.6 in the Appendix, and the electric

fields calculated using Equation 4.5 are collected in Table 4.2.

Single-wavelength absorption changes monitored at wavelengths characteristic for

TiO2(e-)s and the Stark effect were quantified over seven orders of magnitude, from 100 ns to

1 s, shown in Figure 4.6. Care was taken to adjust the incident irradiance such that the long

wavelength absorption measured 2.5 µs after the laser excitation was the same for all the

sensitized materials, such that the number of TiO2(e-)s was constant. The observed kinetics

were non-exponential, but well-modeled by the Kohlrausch-Williams-Watts (KWW)

stretched exponential function:

I(t) = I0 exp[(-kt)β] (4.7)

where I0 is the initial amplitude, k is a characteristic rate constant, and β is inversely

proportional to the width of an underlying Lévy distribution of rate constants, 0 < β < 1.176, 195

The data were fit with β fixed to a value of 0.2 and the abstracted rate constants were kLi+, Na+

= 5 x 104 s-1 and kMg2+, Ca2+ = 5 x 102 s-1.

Table 4.2. Ionic Radii, Spectral Shifts, and Electric Field Strength for Ru(dtb)2(dcb)/TiO2. Cation

Ionic Radii

(Å)a

Electrochemically Accumulated TiO2(e-)sb Photoinjected TiO2(e-)sc

Electric Field (MV/cm) Δν (cm-1) Electric Field (MV/cm)

Li+ 0.76 1.3 53 0.66

Na+ 1.02 1.1 22 0.28

Mg2+ 0.72 1.8 78 0.98

Ca2+ 1.00 2.2 30 0.38

aIonic radii obtained from Shannon, Ref 196. b Electric field change measured after the potentiostatic injection of approximately 20 TiO2(e-) per nanoparticle. cChange in electric field measured 2.5 µs after pulsed laser excitation.


Films.

109

Figure 4.5. Transient absorption spectra obtained 2.5 µs after pulsed 532 nm excitation of

Ru(dtb)2(dcb)/TiO2 in acetonitrile electrolyte solutions containing 100 mM of the indicated perchlorate

salts and 250 mM tetra-n-butylammonium iodide.

Figure 4.6. Single-wavelength transient absorption kinetic data of Ru(dtb)2(dcb)/TiO2 in acetonitrile

electrolyte solutions containing 100 mM of the indicated perchlorate salts with 250 mM tetra-n-

butylammonium iodide observed at the maximum of the Stark effect bleach, ~500-510 nm. Overlaid

in black are fits to the KWW function.

The nature of the cations present in 100 mM acetonitrile electrolytes surrounding a

Ru(dtb)2(dcb)/TiO2 sensitized thin film were varied to test whether they had an influence on

the surface electric field and the dynamics of interfacial charge screening. This was indeed

realized and both the kinetics and the electric field were found to be acutely sensitive to the

nature of the cation. As is often found to be the case in studies of sensitized materials, the

alteration of this one cation variable influenced many properties of the sensitized material


Films.

110

including the interfacial energetics and hence the excited state injection yields. The plausible

origin(s) of these spectral changes are described first followed by a description of the electric

fields and charge screening dynamics.

4.3.1 – Cation Dependent Interfacial Redox Properties.

The adsorption of ions on semiconductor surfaces is known to have a strong influence

on the valence and conduction band edge positions. A well-documented example for metal

oxide semiconductors in aqueous solution is the 59 mV shift of the band edges that

accompanies a factor of ten change in the proton concentration.189, 197-198 The surface

adsorption of alkali and alkaline earth metal cations can have similar effects and are hence

sometimes referred to as ‘potential determining ions’.136, 199-201 In general, cation adsorption

shifts the band edge positions positively on an electrochemical scale away from the vacuum

level. High efficiency dye-sensitized solar cells (DSSCs) utilize anatase TiO2 in non-aqueous

solvents, very often CH3CN, where dramatic energetic shifts have been reported. For

example, the conduction band edge position has been reported to be -2.1 V vs. SCE in 1.0 M

TBAClO4, where the tetrabutylammonium (TBA) cation was reasonably asserted to interact

only weakly with the TiO2 surface, and shifted 1.1 V positive when Lewis acidic Li+ cations

were present.199 Similar shifts have been reported in other non-aqueous solutions.200, 202 The

cation concentration dependence of these shifts is often non-linear and generally one needs to

determine the values experimentally under conditions most relevant to the DSSC.

Spectro-electrochemistry has been widely utilized to characterize the acceptor states of

the mesoporous anatase TiO2 thin films used in DSSCs.43, 192, 203 Reduction of TiO2 results in

a black coloration as well as a blue shift of the fundamental absorption. For a given cation, the

measured spectra were normalizable over the potential range that was investigated, 0.0 to -1.0

V vs. NHE. The extinction coefficients were calculated from the measured absorption spectra

and the amount of charge present in the film as was determined by the charge extraction

technique of un-sensitized TiO2.190-192 The value measured at 700 nm of ε = 930 ± 50 M-1 cm-

1 were within experimental error the same and in good agreement with previously published

values, Figures A4.1-A4.3 in the Appendix.10, 192, 202

While the coloration associated with TiO2 reduction is well known, an assignment of

the underlying electronic transition(s) is not. The blue shift of the fundamental absorption has

been attributed to both an electric field181, 204-205 and a band – filling, i.e. a Burstein-Moss

chapter 2: photophysical and photochemical properties

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